|September 21, 2009
Several AWE concepts rely on long
torque-tubes to bring rotary energy to the ground, but there are severe
scaling limitations. A large torque-tube that reliably resists local
buckling structural failure is far too heavy to fly well.
But water is another story- a roughly 1000 times denser medium than
air. A massive torque tube can easily be neutrally buoyant & work well in
moving water at high megawatt scales. Cheap steel or ferrocement
construction would suffice.
One of Doug
Doug Selsam's AWE schemes puts
ratio turbine blades along a long torque-tube. There are the
standard concerns about the liability of high-speed blades whipping around
people and property while creating a bird- or fish- "blender." But
low aspect ratio turbines (Low AR) like
Dan Parker's ease this concern. Yes, Low AR needs a step-up
gear to match Hi AR rpm, but at common slow water current speeds
(single-digit knots) self-start sooner--and still extract similar power.
Last week I towed a "spiral airfoil" in water, a common kite spin-tail
(Black Lake, Illwaco, WA). The novelty item performed much as in air, but
slower, an old fluid-dynamicist's trick based on dynamic similarity. The
spin-tail developed lots of pull & torque at a knot or two of tow speed.
As speed increased it gracefully furled into a longer narrower screw
geometry that slowed drag increase while maintaining a fairly level
torque/rpm output. Sweeeet. I set another spin-tail in the small stream
behind KiteLab & it spun for hours without a problem in the small shallow
channel that had skunked my High AR hydrofoil experiments.
The spin-tail laid flat on its axis reveals itself as a basic
logarithmic spiral cut from a single piece of untailored fabric. A
plastic whisker is sewn into its outer (leading) edge to stiffen it.
Is there another fluidic device so elegant? Though ancient examples
abound, the oldest extant description of the famous log spiral is by
Descartes. Jacob Bernoulli went so wild over the form proclaiming it the "spira
logarithmic spiral is constrained within a circle it produces a
beautiful curved foil plan that initially broadens from the center then
tapers to a point at the circle's edge. It naturally develops helical
pitch when tensioned along its axis. It also is reversible; just flip
it over & it changes handedness.
When you put a conventional turbine or log spiral (or two or more) at the
end of a torsion tube you can bring the torque aboard a boat & keep the
generator out of the water; a big win. Call it a "swizzle-stick"
turbine. In '07 I played with a tippin' turbine generator in the
Chinook River tidal flow using slow-flight RC propeller.
Two other spirals are worth mentioning. The Archimedean spiral is cruder &
has greater induced drag as a spiral foil. The simple helical spiral has a
constant diameter curl & finds use in bolt threads & as a strake wound
around smoke stacks to break up destructive bluff body oscillation in
A sufficiently long helical strake along a torque tube set axial to
a flow has an amazing ability to exceed the
Betz Limit, which
is based on a disc assumption. It can far surpass the ~60% extraction
limit imposed on a short axis turbine of a given disc area by
progressively entraining more surrounding flow downstream. Envision
long rolling snakes in a water channel developing tremendous torque
without harming aquatic life. In open water, traction kites might tow
these fish-friendly hydropower turbines.
A future note will more fully consider the uses of short "wings" like
strakes, chines, edges, oblique bluff bodies (esp. kite line), and many
kite wings. A short "ugly" wing can have reasonable L/D at low Re & the
high drag at high
(Re) is useful for progressive stability mechanisms. Wing-loading
will be explained as a simple general predictor of AWE reliability.
~ Dave Santos